Method of improving plating distribution of elnisil coatings

ABSTRACT

An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semi-epitrochoidally aligned grooves respectively machined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

United States Patent 1 Cordone et al.

[ 1 Oct. 28, 1975 METHOD OF IMPROVING PLATING DISTRIBUTION OF ELNISILCOATINGS Mich.

[731 Assignee: Ford Motor Company, Dearborn, Mich.

[22] Filed: July 1, 1974 21 Appl. No.: 484,729

Related US. Application Data [62] Division of Ser. No. 413,154, Nov. 5,1973, Pat. No.

FOREIGN PATENTS OR APPLICATIONS 4/1944 Germany 204/DIG. 7

OTHER PUBLICATIONS Leadbeater, Metal Industry, Apr. 28, 1944, p. 266.

Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm-Joseph W.Malleck; Keith L. Zerschling [57] ABSTRACT An electroplating system forcathodically plating an epitrochoidally shaped internal surface of arotary engine housing. An anode assembly is provided which is comprisedof a perforate walled container of titanium metal or other anodicallyinert metal to which a voltage potential can be applied; the basketcontains anode pieces such as nickel which are shaped to be in intimatecontact with each other during the plating operation. The perforatewalls of the anode container is shaped from flexible expanded titaniumsheet metal interfitted within semi-epitrochoidally aligned groovesrespectively maehined into titanium plates forming the ends of the anodeassembly. The anode walls are thus shaped substantially complimentary tothe epitrochoid configuration of the cathode but have a predetermineddeviation adjacent the nodes of the trochoid for insuring a uniform butheavy coating thickness under high speed electroplating conditions.

3 Claims, 4 Drawing Figures US. Patent Oct.'28, 1975 SheetlofZ 3,915,835

METHOD OF IMPROVING PLATING DISTRIBUTION OF ELNISIL COATINGS This is adivision of application Ser. No. 413,154, filed Nov. 5, 1973, now U.S.Pat. No. 3,891,534.

BACKGROUND OF THE INVENTION It is generally well known in electroplatingthat the density of current flow will be uneven at the sharp edges orcontour changes on the object to be plated. This phenomenon involves aproposition that there is increased plating resulting from increasedcurrent density at any outstanding contour, while the opposite effectwill take place at depressions. In the latter case, the

density of current flow becomes less than the average density of platingcurrent over the full area being treated. This problem becomesexaggerated when an article to be plated has a compound curvature, suchs in an epitrochoid, where the cathode is able to receive current throwfrom two different zones or anode locations due to the reverse orcompound curvature. Accordingly, certain areas will be unduly thickbecause of the throwing power which is multiplied in some areas.

In applications such as a functional coating for a wear surface of aninternal combustion engine, i.e. the internal rotor housing surface of arotary internal combustion engine, the need for uniformity in thecoating is extremely severe. The efficient electroplater not only seeksto obtain uniform thickness in such applications, but the plating mustbe of good sound density throughout; the latter will be degraded as aresult ofinappropriate bath chemistry, electrode spacing, and change ofthe anode or cathode area during the plating process.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide an anode assembly useful in an electroplating system forcathodically plating an article having a compound or reverse curvature,the anode assembly being particularly adapted to maintain a propercurrent throw relationship so that a uniform thickness and density ismaintained throughout the plated surface of said article.

Another object of this invention is to provide a semiconforming anodeassembly or apparatus for use in an electroplating system of the typewhich is adapted to deposit a significantly heavy functional coating ona non-uniformly curved surface.

Features pursuant to the above objects comprise the use of an anodeassembly having a foraminous or perforate sheet metal titanium wallshaped in a predetermined unique configuration and varied from the shapeof the cathode at selected locations. The wall is retained by end platesformed of the same material but solid. The cross section of theforaminous wall is defined so that it is semi-conforming with respect tothe shape of the cathode; the anode assembly progressively becomes morespaced from any portion of the cathode article which has a reverselycurved portion, the progression of spacing increasing to a locationintersected by radius of the reversely curved portion passing throughthe mid-point thereof.

Still another object of this invention is to provide a novel and uniquemethod for fabricating an anode assembly which will have a defined crosssectional configuration with a semi-conforming relationship to thecathode, a continuous wall of the assembly being fabricated of perforatesheet metal, such as titanium.

BRIEF SUMMARY OF THE DRAWINGS FIG. 1 is a schematic illustration of anelectroplating apparatus having a stacked series of cathodicallyconstituted articles for plating, and an anode assembly disposed withinthe interior of said seriesof cathode articles;

FIG. 2 is a plan. view of the apparatus of FIG. 1, shown similarly in asomewhat schematic manner;

FIG. 3 is a highly enlarged schematic layout of the cross sectionalconfiguration of the anode assembly and the inner wall of the cathodicarticle to be plated;

FIG. 4 is an exploded view of the basic elements which interfit to formthe cathode assembly according to the method of this invention.

DETAILED DESCRIPTION Turning now to the drawings and particularly FIGS.1 and 2, there is schematically illustrated a preferred mode for ananode assembly and plating system according to this invention. Anelectroplating tank A is provided to contain an electrolyte B, such asan aqueous solution of nickel sulfamate containing inert particles ofsilicon carbide. Typically the bath may contain about 600 grams/liter ofnickel sulfamate, about grams/liter of silicon carbide with a mesh sizeno greater than 400, about 2.5 grams/liter of a stress reliever such assaccharin, about 19 grams/liter of nickel chloride, and about 45grams/liter of boric acid a a)- I A'cathode assembly C is disposed inthe electrolyte consisting of several cathodic articles 10 eachconstituting a cast aluminum rotor housing useful as an element of arotary internal combustion engine. The rotor housings are annular andmust have a highly wearresistant epitrochoid surface 11 on the interiorthereof and against which apex seals or other moving parts of a rotaryengine must hear. The housings are separated, one from the other, byspacers 12 which may act as shields and prevent plating on the sidefaces 10a of the rotor housings. Such spacers can be formed aspolypropylene or nylon sheets and have an inner edge 13 which isrecessed from the interior surface 11 ofeach rotor housing.Alternatively, the spacers may be arranged as cathode elements which fittightly between the housings and which draw current around the edges ofthe housings to overcome the problem of exaggerated thickness at suchedges; again the spacers would be recessed as illustrated.

At the upper and lower ends of the stack of housings and spacers, thereis employed a rigid annular shield 14 for the top and bottom faces 15and 16 respectively. Each shield should be a plate comprised of aluminumcoated with silicone rubber which stays clean and does not draw plating.Plates are supported by a harness (not shown) which facilitates thelowering and the raising of the entire cathode assembly from theelectrolyte. The harness should similarly be coated so as to have aninert outer surface.

An anode assembly D is employed which is of a semiconforming typewherein only a portion of the anode is adapted to be proportionedidentical to the cathode surface 11 to be plated; other portions aredesigned to progressively deviate from such configuration. The anodeassembly, here, is a basket made from expanded titanium sheet metal (ormay be woven from titanium wire). The walls 19 are foraminous and thebottom and top walls 20 and 21 are each a solid titanium plate.Resilient or elastic neoprene bands 22 may be mounted about the anodewall 19 to mask off or block the current throw in certain predeterminedelevation zones along the anode assembly, particularly those areas wherethe edges of the cathode article would promote an uneven distribution.The masking also blocks off current throw to the spacing between thehousings. Such masking is unnecessary if cathodic spacers are utilizedas mentioned earlier. Active anode pieces 23, such as nickel, arecollected and stacked in the basket for intimate interengagement witheach other and with the basket.

An anodic film is formed on the titanium basket which affords corrosionresistance and electrical insulation, the basket thereby being renderedanodically inert. The titanium acquires a thin-dense inert oxide filmwhich is chemically resistant to acidic electrolytes and has a highelectrical resistance. The current density, of course, is controlled bythe configuration of the titanium basket even though the nickel anodepieces therein are the active anode metal. Current will pass between thebasket and pieces at a contact point between the anodic film and thenickel pieces; this is so even though the film on the titanium is anelectrical insulation.

To realize the objects of this invention, the anode walls 19 are definedwith a predetermined variation from the nodes 24 of the epitrochoidconfiguration of cathode wall 11. Such nodes or segments have a reversecurvature relative to the uniform curvature of the remaining portions 25and 28 of the epitrochoid; in cross section, the portions 25 maysubstantially be arcs of circles. The cross sectional configuration ofwall 19 has a pair of uniform arcuate segments 26 and 27 which aredirectly proportional and aligned with the segments 25 and 28respectively of wall 11. At stations on wall 19, substantially adjacentthe extremities of the reverse curvature segments 24, varying arcuatesegments 30 and 31, each of which may have a different radius from thatof the uniform segments, are employed. Each of these varying arcuatesegments substantially continue or extend the curvature from each of theuniformly curved segments until they meet at ajuncture 32 which is onthe minor axis of the epitrochoid, or in other terms, has a radius 33 ofthe reverse curvature segment 24 passing therethrough and through amidpoint 34 of the reverse curvature. In this manner the total segment30 or 31 is each comprised of two arcs meeting at an abrupt juncture andthereby rendering the combination as varying in curvature.

As shown in FIG. 3, the uniform segments 27 and 26 are formed fromcricles which overlap. It is possible that for some types ofepitrochoids or compound cathode surfaces, the circles should be madetangent. In any event, the deviation (distance 36 distance should beprogressively varied according to the relationship whereby the deviationis inversely proportional to VC.D., where CD. is current density,provided such factors as the conductivity of the solution andtemperature are constant.

Method of Making Anode Assembly A preferred method mode according tothis invention with respect to FIG. 4, comprises:

1. Prepare solid titanium end plates 44 and 45 with identical continuousgroove 46 and 47 respectively, each groove defining a semi-conformingconfiguration to that of the cathode surface. In this case, thesemiconforming configuration comprises two uniform arcs 40 and 41connected by varying segments 42 and 43. The varying segments areadapted to render a predetermined deviation away from the cathodesurface at these areas to promote uniform plating thickness.

2. Assemble a flexible web of titanium expanded sheet metal (having amesh size no greater than with the longitudinal edges 50 and 51 of theweb in the grooves 46 and 47 respectively. The web is overlapped uponitself at a seam 52 to define a sleeve-like wall with a uniform crosssection reflecting the uniformity and deviations of said grooves.

3. Locate the ends of posts 53 and 54 in mating seats 55 in the endplates to effect a strong stable joint between said plates.

4. Stitch the seam 52 with titanium wire, and fill the assembly with acollection of nickel anode pieces.

5. Provide suitable electrical means for applying a potential to theweb.

We claim as our invention:

1. An anode basket for an electroplating process wherein the cathode hasa compound curvature with a transition point separating the differentcurvature thereof, said anode basket having a generally complimentarycompound curvature except opposite said compound curvature to render afirst anode effect on said cathode, said anode curvature beingprogressively deviated up to a maximum opposite said transition point,said deviation occurring where the anode effect on any one point on thecompound curvature of said cathode is increased beyond said first anodeeffect, said deviated curvature at said maximum terminating in a sharpapex.

2. An anode basket adapted to fit within and promote electroplating ofan inwardly directed epitrochoidally shaped cathode, said cathode havinglobes which present a compound curvature with reversely curved segments,said anode basket having first portions adapted to be complimentary inshape to corresponding segments of said cathode and to reside at auniform distance therefrom, said anode basket further having at leasttwo second portions residing opposite each lobe and differing from thecurvature of said lobes, said second portions having a radius ofcurvature smaller than any radius defining the curvature of said lobes,said second portions meeting at a sharp juncture lying on the minor axisof said epitrochoidally shaped cathode.

3. An anode basket as in claim 2, in which said second portions arearranged to be substantially tangent to each other, said juncture lyingon a line of tangency.

1. AN ANODE BASKET FOR AN ELECTROPLATING PROCESS WHEREIN THE CATHODE HASA COMPOUND CURVATURE WITH A TRANSITION POINT SEPERATING THE DIFFERENTCURVATURE THEREOF, SAID ANODE BASKET HAVING A GENERALLY COMPLIMENTARYCOMPOUND CURVATURE EXCEPT OPPOSITE SAID COMPOUND CURVATURE TO RENDER AFIRST ANODE EFFECT ON SAID CATHODE, SAID ANODE CURVATURE BEINGPROGRESSIVELY DEVIATED UP TO A MAXIMUM OPPOSITE SAID TRASITION POINT,SAID DEVIATION OCCURRING WHERE THE ANODE EFFECT ON ANY ONE POINT ON THECOMPOUND CURVENTURE OF SAID CATHODE IS INCREASED BEYOND SAID FIRST ANODEEFFECT, SAID DEVIATED CURVATURE AT SAID MAXIMUM TERMINATING IN A SHARPAPEX.
 2. An anode basket adapted to fit within and promoteelectroplating of an inwardly directed epitrochoidally shaped cathode,said cathode having lobes which present a compound curvature withreversely curved segments, said anode basket having first portionsadapted to be complimentary in shape to corresponding segments of saidcathode and to reside at a uniform distance therefrom, said anode basketfurther having at least two second portions residing opposite each lobeand differing from the curvature of said lobes, said second portionshaving a radius of curvature smaller than any radius defining thecurvature of said lobes, said second portions meeting at a sharpjuncture lying on the minor axis of said epitrochoidally shaped cathode.3. An anode basket as in claim 2, in which said second portions arearranged to be substantially tangent to each other, said juncture lyingon a line of tangency.